The long-term goal of our research is to determine the fundamental mechanisms that regulate microtubule dynamics during diverse cellular functions. A central unanswered question is how microtubule dynamics are spatially controlled during interactions with cellular targets or attachment sites. Specifically, microtubule interactions with the cell cortex play important roles in processes such as cell polarization, cell migration, spindle positioning, and cytokinesis. Kinesin-8 is a conserved family of motor proteins and an important class of microtubule regulators essential for regulating kinetochore-microtubule dynamics and successful mitosis in human cells. Currently, the mechanism(s) used by Kinesin-8 proteins to control microtubule dynamics at cellular target sites is not clear. Budding yeast has been a productive model for elucidating mechanisms underlying microtubule interactions with the cortex. In these cells, Kinesin-8 (ScKin8) regulates cortical- microtubule interactions required to position the spindle during asymmetric cell division. ScKin8 is a dual activity motor that combines conventional motility with a plus-end specific depolymerase activity. Based on our preliminary data, we will combine yeast genetics and cell biology, live cell imaging, biochemical reconstitution, and in vitro real-time observation of dynamic microtubules by evanescent wave fluorescence microscopy to address the following Aims: 1) To define how ScKin8 alters microtubule dynamics and interactions with the cell cortex, 2) To determine how the regulatory tail domain spatially controls ScKin8 activity, and 3) To dissect the molecular features and activities of ScKin8 that underlie its biological function at the cell cortex. These findings will likely reveal conserved mechanisms and strategies used to control microtubule dynamics at other sites where Kinesin-8 functions in yeast and higher organisms.